Alkenes push-pull

Bis [(trifluoromethyl)thio] acetaldehyde (83a) has been prepared from an enam-ine precursor (84), although refluxing in aqueous ethanolic HCl is required to effect this reaction.The aldehyde is less stable than its enol tautomer (83b), and many reactions typical of aldehydes fail. For example, addition of aqueous silver nitrate immediately yields the silver salt of (83b), rather than giving precipitation of (elemental) silver. The (trifluoromethyl)thio substituent has pseudohalogenic character and, together with the hydroxy group, stabilizes the alkene tautomer in the manner of a push-pull alkene. The enol-aldehyde equilibrium mixture in acetonitrile shows an apparent of 2.6 when titrated with aqueous hydroxide. 

X. Y = NH. O, S. NHJ Figure 1 Push-pull alkenes with a heterocyclic moiety. 

Vinamidines and Vinamidinium Salts — Examples of Stabilized Push-Pull Alkenes D. 

Push-Pull Alkenes as Reactive Synthesis Elements for Heterocycles H. Kristen and K. Peseke, Wiss. Z. Wilhelm-Pieck-Univ. Rostock, Math.-Naturwiss. Reihe, 1976, 25, 1123-1130. 

Resin-bound amines can be converted into imines [710,711] or enamines by reaction with carbonyl compounds (Entries 6 and 7, Table 3.39). Resin-bound enamines have also been prepared by Michael addition of resin-bound secondary amines to acceptor-substituted alkynes [712], by Hg(II)-catalyzed addition of resin-bound secondary amines to unactivated alkynes [713], by addition of C-nucleophiles to resin-bound imino ethers [714], and by chemical modification of other resin-bound enamines [712,713,715], Acceptor-substituted enamines ( push-pull alkenes) are not always susceptible to hydrolytic cleavage by TFA alone and might require aqueous acids to undergo hydrolysis [716]. 

The addition of organometallic reagents to push-pull alkenes is a synthetically important process in which retention of an alkenic unit in the product occurs this process complements the direct conjugate addition of organometallic reagents to activated alkynes (Section 1.2.5) which is complicated primarily... 

Nitroenamine derivatives are typical push-pull alkenes, and have been widely used in organic syntheses [3,30,31]. Although their synthetic utility is quite high, functionalized ones are not often employed because of their difficult preparation. Hence, the aminolysis of pyrimidinones 2 and 3 is regarded as a new preparative method for functionalized nitroenamines 10 and 11. 

Alkenes substituted with an electron-withdrawing (Z) and an electron-donating group (X) will be less reactive than the unsubstituted alkene if the substituents are vicinal (Z-C=C-X push-pull alkenes), but will generally become highly reactive when these two substituents are bound to the same carbon atom. The latter type of alkene (ZXC=CR2 1,1-captodative alkenes) readily dimerize to yield cyclobutanes and can react with 1,3-dienes to yield products of [2 + 2] and/or [2 + 4] cycloaddi-... 

Vinamidines are examples of push-pull alkenes, and other push-pull alkenes R1COCR2=CR3X (X = SR, OR) also react with o-phenylenedi-amine to give benzodiazepines, e.g., Scheme 15 (85JHC405 96TL9155). 

Alternatively, the substituents A and B may stabilise a cationic centre on one side and the substituents C and D an anionic centre on the other 2.102. Alkenes having donor substituents at one end and acceptors at the other are called push-pull alkenes, and the barriers to rotation are indeed lowered,137 with the enamine system of the alkene 2.107 having a barrier of 66 kJ mol-1 (16 kcal mol-1).138 More subtly, the substituents in the allene 2.108 enable the phenyl and the methyl groups to exchange places rapidly, with coalescence of... 

A very recent report highlights the cross-coupling of Fischer carbene complexes with ethyl diazoacetate to give push-pull alkenes such as 106 at ambient temperature using 15 mol-% CuBr as a catalyst (Scheme 46) [98] alkyl-, alkenyl- (105) and arylcarbenes provided comparable good chemical yields. 

In tropolones electronic interaction between the functional groups is facilitated by the planar geometry of the molecule. Both tropolones and B-diketones (in their enol form) are examples of push-pull alkenes and contain enol moieties which lead to ready reactivity towards electrophiles at the 3-, 5- and 7-positions of tropolones as at the 2-carbon atom of 1,3-diketones, which in the latter case had been recognised many years earlier. 

The location of two electron-donating, amino groups on one carbon, and two electron-withdrawing, nitro groups on the other carbon should make 1,1-diamino-2,2-dinitroethene a good example of a push-pull alkene [31], with strong polarization of the double bond (Scheme 5). 

D. Lloyd and H. McNab, Vinamidines and Vinamidinium Salts. Examples of Stabilized Push-Pull Alkenes , Angew. Ghent. Internat. Edn., 1976, 15, 459. 

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